The sample coil has been known to be of critical importance to NMR sensitivity since the initial experiments more than 50 years ago, and solenoids of various designs have been consistently selected for numerous applications because of their advantages in Q and filling factor .eta..sub.F. Twenty years ago, David Hoult's classic work showed that in his experiments, best performance (.eta..sub.F Q product) was generally obtained with round wire of diameter such that the space between turns was about half the diameter of the wire. Three years later, I showed that closely spaced flattened wire was preferred because of higher .eta..sub.F and improved B.sub.1 and B.sub.0 homogeneity (without compensation), although Q may be slightly lower, and this geometry became standard practice for more than a decade (see `Probe Design and Construction`, in The Encyclopedia of NMR, Vol. 6, Wiley Press, 1996). Prior-art multi-turn rf sample coils utilizing flattened conductors have utilized pure copper and have always oriented the conductors such that the major cross-section axis is parallel to the coil axis and the minor cross-section axis is perpendicular to the coil axis. This configuration is denoted `flat-wrapped`.
About four years ago, with the advent of preliminary rf magnetic finite element analysis (M-FEA) software, we discovered that current density over the surface of the wire was more concentrated along the inner edges at the ends of the solenoid than had previously been expected. Hence, we found some improvement in Q from making solenoids from round wire that had been flattened only at the central turns, as noted in a copending application on thermal buffering of transverse resonators and in the above referenced article. However, Hoult's original design is often still regarded as optimum for Q, as evidenced in U.S. Pat. No. 5,539,315, for example.
Other prior art low-inductance NMR solenoids utilized two parallel round wires for similar advantages in Q.sub.0 and B.sub.1. Numerous electro-mechanical transducers, especially loudspeaker voice-coils and moving-coil motors and generators, have often utilized flattened edge-wound conductors (major cross-section of conductor perpendicular to coil axis) for better impedance match to the desired source and for improved mechanical performance. However, one of the objectives of all such prior edge-wound coils is also to achieve nearly 100% filling of a field gap with copper, so space between turns is the minimum required for electrical insulation and is typically a small fraction of the minimum dimension of the flat wire. Electromagnetic skin depths in prior edge-wound multi-turn transducer coils, at least at the minimum frequency of intended use, are usually very large compared to the major cross-section dimension of the wire.
As disclosed in the referenced copending application, there are numerous NMR applications, particularly in VAS or DAS at high fields (see `Solid State Probe Design`, in The Encyclopedia of NMR, Vol. 6, Wiley Press, 1996), where it is advantageous to utilize a transverse coil for .sup.1 H decoupling in combination with a solenoid for the low frequencies. When a transverse coil is present (or when transverse access is needed for other reasons), it is necessary to increase the relative space between turns of the solenoid to improve performance of the transverse coil (or for other access). Prior work had shown round wire to be preferred over flat-wrapped wire under these conditions. However, with solenoids of two to six turns, increasing the relative space (either with round or flat-wrapped conductors) between turns causes a sharp drop in .eta..sub.F and Q and degradation in B.sub.1 and B.sub.0 homogeneity, although the effects become less significant with increasing number of turns. The drop in Q is considerably greater than predicted by available M-FEA software, suggesting that currents are less concentrated at edges than indicated by current software or rf resistivity and skin depth are larger than expected. Hence, the degree of the advantage that is found with edge-wound transparent rf solenoids where skin depths are very small compared to wire dimensions and where wire widths are comparable to turns spacing is quite surprising. B.sub.0 inhomogeneity is exacerbated by the edge-wound coil, but the effects may be reduced by careful magnetic compensation. Prior-art compensation methods include electroplating copper over zinc-plated aluminum and co-extruding round copper over round aluminum wire.
The single-turn rf solenoid, normally referred to as a split-ring or loop-gap resonator (see `Surface and Other Local Coils for In Vivo Studies`, by James Hyde, in The Encyclopedia of NMR, Vol. 7, Wiley Press, 1996,), has sometimes been configured as an open structure utilizing a number of narrow bands paralleled along the gap where the capacitors are inserted, as by Chowdhury et al in U.S. Pat. No. 5,363,845. This makes it more transparent to transverse gradient or rf fields, as would often be present in MRI, although the design by Chowdhury et al has inadequate transverse transparency for optimum performance under some conditions. The prior-art transparent-loop-gap resonators have used round wire and ribbon, both flat-wrapped and edge-wound, as the edge-wound configuration was found to have lower parasitic capacitance to the sample. Prior-art edge-wound transparent-loop-gap resonators for large samples have had space between bands much greater than five times the major cross-sectional dimension b of the bands, with b less than 5% of the coil inside radius r. The relatively low .eta..sub.F and Q.sub.0 of prior-art edge-wound loop-gap coils is sometimes inconsequential in applications where sample losses dominate, although higher B.sub.1 homogeneity is usually preferred. Magnetically coupled loops (as disclosed by Banson et al in `A Probe for Specimen Magnetic Resonance Microscopy`, Invest. Radiology, 27, 2, 157-164,1992), consisting of two magnetically coupled, planar, loop-gap resonators etched on dielectric substrates with b comparable to or even larger than r, have also been used for MR microscopy. However, their distributed capacitors negate the primary objective of segmentation--i.e., reduction of electric fields in the sample. Multi-turn planar foil spirals have also been used as surface coils.